1. Field of the Invention
This invention relates to color diffusion transfer photographic elements and, in particular, to color diffusion transfer photographic elements having a mordant layer for forming color images, said layer containing a seed polymerized cation latex.
2. Description of the Prior Art
It is well known to use a variety of polymer materials as mordants for preventing the transfer of dyes in the field of photographic techniques. It is also known that a polymer having a quaternary nitrogen atom is useful as a mordant for dyes having a group providing an anion by dissociation, such as a sulfonic acid group, a sulfonamido group, etc., particularly when used as mordants for forming color images.
When a polymer is used as a mordant for forming color images, it is necessary for maintaining high image density to prevent dyes from escaping or diffusing from a mordant layer to other layers, and for this purpose, a polymer having excellent mordanting properties is required. Thus, polymer mordants having properties of receiving dyes and strongly holding images formed have been widely investigated, and it has been clarified in U.S. Pat. No. 3,898,088 to be desirable (for having excellent mordanting properties for dyes) that the polymer mordant be insoluble in water and have the recurring unit represented by formula (II) ##STR2## wherein R.sub.1 and R.sub.2 each represents hydrogen or an alkyl group having from 1 to 6 carbon atoms; R.sub.3, R.sub.4, and R.sub.5 each can represent an alkyl group having from 1 to about 20 carbon atoms; and X.sup..crclbar. represents an anion; said polymer mordant being insoluble in water by controlling: (a) the total carbon numbers of R.sub.3, R.sub.4, and R.sub.5 ; (b) the amount of an ethylenically unsaturated comonomer, if any; and (c) combinations of (a) and (b).
When a polymer mordant shown by formula (II) is insoluble in water, steps for dissolving the polymer mordant in an organic solvent and then coating the organic solvent solution are necessary for incorporating the polymer in photographic elements. However, in the case of coating such a solution of the polymer mordant in an organic solvent, various difficulties are encountered, as described below, and hence it is difficult to produce stable photographic elements. Such difficulties include:
(1) Explosion-proof coating equipment may be required by law.
(2) The application of the organic solvent solution onto other photographic layers (e.g., a silver halide emulsion layer) for forming multilayer elements is very difficult.
(3) The selection of an effective hardening agent for the layer is greatly restricted.
(4) It is difficult to incorporate a light-fading prevention agent therein to prevent fading of transferred dyes.
In order to overcome such difficulties, it is preferred that the solvent for coating compositions be an aqueous medium.
As a technique for coating a water-insoluble polymer in an aqueous system, a method is known wherein the polymer is coated as an aqueous latex thereof. It is, however, difficult to prepare a polymer cation latex having excellent mordanting properties by conventional methods. That is, for preparing a polymer latex from a water-insoluble monomer represented by formula (I) below, containing a quaternary nitrogen atom, a method is of emulsion-polymerizing the water-insoluble monomer using a water-soluble polymerization initiator in the presence of a cationic surface active agent and/or a nonionic surface active agent, or in the presence of a water-soluble polymer such as polyvinyl alcohol, gelatin, etc., or in the presence of such a water-soluble polymer and the above-described surface active agent(s). Monomers according to formula (I) are represented by ##STR3## wherein R.sub.1 and R.sub.2 each represents hydrogen or an alkyl group having from 1 to 6 carbon atoms; R.sub.3, R.sub.4, and R.sub.5 each can represent an alkyl group having from 1 to 20 carbon atoms, an aralkyl group having from 7 to 10 carbon atoms, or any two of R.sub.3, R.sub.4, and R.sub.5 together can form a ring; and X.sup..crclbar. represents an anion. The aralkyl group can be substituted by a halogen atom or a nitro group.
The monomer employed in such an emulsion polymerization can be:
(i) a monomer of formula (I) alone, or
(ii) a mixture of a monomer of formula (I) and at least one vinylic monomer other than that of formula (I) which are to be copolymerized (in this case, it is preferred that the vinylic monomer other than that of formula (I) be insoluble in water).
When the polymer latex is to be used as a mordant, it is desirable that the content of quaternary nitrogen atoms per the unit weight of the polymer be as high as possible. From this viewpoint, the use of a monomer of formula (I) alone, as in item (i) above, is most preferable, but in this case it is very difficult to select conditions for preparing a stable polymer latex of fine particles which have a polymer concentration of higher than 5% by weight (which is reuired in practical use) without forming coagulates of the polymer latex and which is capable of providing coatings having a transparency sufficient for practical use. Thus, no totally practical method is known in the art for producing a polymer latex.
In the case of using a mixture of a monomer of formula (I) and at least one vinyl monomer other than that of formula (I) which are to be copolymerized, as in item (ii) above, the conditions for producing a stable polymer latex without coagulation of the polymer may be relatively easily selected, but when the proportion of the monomer of formula (I) in the copolymer is in the range of from 50 to 99 mol%, which is preferable for a mordant, the mean particle size of the polymer latex formed becomes large, thereby greatly reducing the transparency of coatings formed therefrom. The tendency of increasing the mean particle size is increased dramatically as the proportion of the monomer of formula (I) nears 100%, which is a particular drawback of the method.
It is, furthermore, difficult to prevent the formation of a low molecular weight oligomer (degree of polymerization of from 2 to 10) in the copolymer latex wherein the composition ratio of the monomer of formula (I) is in the range of from 10 to 95 mol%. In a color diffusion transfer system, the intermixing of such low molecular weight components (including monomers, oligomers, etc.) is undesirable, since these low molecular weight components leave a mordant layer composed of gelatin and a polymer mordant and diffuse from the layer into an adjacent white reflecting layer or other layer and they capture dyes diffused therein from photosensitive silver halide emulsion layers, whereby the amount of dyes reaching the mordant layer is reduced to greatly deteriorate the density of dye images formed.
Other methods of producing cationic high molecular latexes for photography are described in Japanese Patent Application (OPI) Nos. 73440/76 and 45231/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), but these methods have drawbacks as described below. That is, in the methods disclosed in Japanese Patent Application (OPI) Nos. 73440/76 and 45231/78, a polymer latex is prepared using a vinylic monomer having a group causing a reaction with a tertiary amine to form a quaternary nitrogen atom, such as, for example, vinylbenzyl chloride, and thereafter the polymer latex is reacted with a tertiary amine using an auxiliary solvent, to provide a cationic polymer latex. More specifically, the cationic polymer latex is produced in these methods by the polymerization of a monomer shown by formula (III) ##STR4## wherein R.sub.1 and R.sub.2 each represents hydrogen or an alkyl group having from 1 to 6 carbon atoms and Z represents a halogen atom, and the subsequent quaternarization reaction with a tertiary amine shown by the formula (IV) ##STR5## wherein R.sub.3, R.sub.4 and R.sub.5 each represents an alkyl group having from 1 to 20 carbon atoms, an aralkyl group having from 7 to 10 carbon atoms (wherein the aralkyl group can have a halogen atom or a nitro group as a substituent); or said R.sub.3, R.sub.4 and R.sub.5 may combine with each other to form a ring.
However, in these methods a cationic polymer latex can be relatively easily prepared only when the tertiary amine has a relatively short alkyl chain, but resulting cationic polymer latex does not provide good mordanting properties. In other cases, when a tertiary amine of formula (IV) having a long alkyl chain (e.g., trihexylamine, etc.) is used in order that the resulting cationic polymer latex is insoluble in water (i.e., to provide superior mordanting properties), the methods described above encounter the following drawbacks:
First, when a water-insoluble tertiary amine is used, the quaternarization reaction does not proceed and the latex forms coagulates.
Second, for reacting a tertiary amine which is reluctant to cause quaternarization reaction, with a polymer, a method must be employed in which an auxiliary organic solvent (e.g., methanol, etc.) capable of dissolving the tertiary amine and being miscible with water is used, but in this case it is very difficult to find an auxiliary organic solvent fitting the purposes without having adverse influences on the stability for dispersion of a percursor latex (i.e., polymer latex formed from the monomer of formula (III)).
Third, the reaction of the tertiary amine and a polymer is essentially a macromolecular reaction, and hence even if conditions for reacting these reactants are found, it is still impossible to reach the theoretical 100% reaction. In particular, when a tertiary amine having a long alky chain is used, the reaction is not favored, and it is impossible to react more than about 80% of the halomethyl group, which results in difficulty in obtaining a cationic polymer latex having excellent mordanting properties.
Fourth, when a large amount of tertiary amine is added to increase the efficiency of the quaternarization, the tertiary amine remaining in the polymer latex formed adversely affects the stability for the dispersion of the polymer latex, and when such a polymer latex is used for photographic elements, it can cause fogging of silver halide emulsions.
Fifth, the halomethyl group remaining in the polymer latex formed has a very high reactivity, and the presence of such a group not only reduces the stability of the polymer latex by the occurrence of cross-linking, but also causes a hydrolysis to release hydrochloric acid, which makes it difficult to control the pH of the polymer latex. Furthermore, when the polymer latex is used as a mordant for photography, the presence of the halomethyl group causes a reduction in image density, an increase in stain, and so forth.